I have searched for threads on rear ends codes for 69 z28s and I am having trouble. My rear is stamped BN 10 16 G2 and the casting date is J 1 9 with 3969341 casted as well. Is the BN 3.55 Non Posi ratio possible for a Z? (TT date 11A). Thanks for your help!

Yes, it's possible.. although not the std ration (3.73 is std). Posi was an option, and not mandatory for Z28s or any other model I don't believe... although I'd bet 99% of Z28's had Posi... 3.55 would be good for the highway, but would likely be difficult around town given the 302 characteristics...

I had a Z with 3.55 gears and a M-21, the car was such a dog off the line, until you were rolling at about 20mph, then you were good to go. In fact I had to keep a close eye on the tach because the motor was still pulling well into the 7000 rpm range. I have have the 3.73 which is a lot better in my current Z, and I would say a 4.10 or even 4.55 would be more ideal a set up for the 302 4 speed configuration.

A 3.55 M20 (8.946) would have a steeper effective first gear than a 3.73. M21 (8.206) It is very close to a M21 4.10 effective ratio of (9.02)

The close ratio box was really designed for road racing, where there is more need for tighter ratios in order to keep the engine in the peak of the curves through the differing turns and elevation changes. The availability of a slightly different gear ratio between 1st 2nd and 3rd in drag racing has little over all effect on the ET, where as when you are shifting 15 times per lap and you can pick up a slight advantage in pull coming out of the turn X every lap then tailoring ratios will pay off significantly.

I wonder how often back then (when they were road racing) they would swap out their rear end gears and transmissions for better ratios fro a particular track. I would think the M-21 with the 4.10 may have been a good combo for some tracks, but it seems like it wouldn't have been a one size (ratios) fits all week in and week out. Would the M-20 ratio have ever been preferred for a specific track with a special rear end ratio? Maybe Jon Mello (trans am guru) would have some insight here ?

I've not ever heard of a road racing team using the M20. I'm not saying it couldn't have happened but I'm not aware of it.Because of the relatively small displacement, the torque range is narrower and higher up the rpm scale. If there is a largerjump between each gear ratio, the engine speed could fall out of the optimal range and that would have an adverse effecton lap times. You are rarely using first gear in a road race except for the start or leaving the pits. Ford did homologate a lotof different gear ratios for the transmissions but they had a huge amount of money that they openly threw at their racingprogram and Chevy was not allowed to race at that time. Ford would rent track time ahead of each Trans-Am event and dialin a car in terms of tranny ratios and rear axle ratios (plus spring rates, etc) and Penske would do this as well but I've justnever heard of an M20 being used. In 1970, the SCCA allowed race teams to use rear axle and transmissions from rival manufacturers. This allowed the Chevy teams to use the Ford rear axles and transmissions and of course it is much easierto make a rear axle gear change on the Ford axle than on the Chevy. Prior to that, most independent Chevy racers were probably just running the same rear axle ratio regardless of the track and doing the best they could. Penske could affordto make a gear change in his rear axles or have a few spare trannys and axles on the truck but most of the independentscouldn't afford that.

I would say in '67 that the majority of guys were using the M21 as that was the only tranny that was offered in the Z-28sthat year. I know for a fact that was the case with my gold race car. As each year went by in Trans-Am racing from '67 to'72, I would believe that the usage of the M22 went up.

In 1970, the SCCA allowed race teams to use rear axle and transmissions from rival manufacturers. This allowed the Chevy teams to use the Ford rear axles and transmissions and of course it is much easierto make a rear axle gear change on the Ford axle than on the Chevy. Prior to that, most independent Chevy racers were probably just running the same rear axle ratio regardless of the track and doing the best they could.

I never knew about that rule change.My father's mentor was an old German guy that ran dirttracks back in the 50's. Guy was a mechanical whiz. He could change the axle gears in about 10-15 minutes, including setting up the lash. Yikes. I bet once you had it dialed in, you knew which shims were needed for which gear sets.

Another thing to remember when road racing is flywheel weight and center of mass... removing a few ounces from the rotating assemblies via-aluminum flywheel and dual disk clutch is worth as much in lap times as removing 100 pounds of car mass. I read the formula in a technical automotive engineering & design book I have had for years, however reciprocating assemblies in engines are one of the best ways to gain power... you are accelerating each piston from a dead stop 2 times per revolution... think about that. I worked for Gas Rhonda's Cammer engine builder in the early 90's and may have overheard a thing or two...